Generally, the principle of measuring temperatures distributed in a measured optical fiber using a back scattering light of the optical fiber is as described below. If an optical pulse of an excitation light source enters into the measured optical fiber, a scattering light is generated in the optical fiber, and a part of the scattering light is fed back into the incoming end of the measured optical fiber, and thus the back scattering light is formed. Most of the back scattering light is a Rayleigh scattering light having a wavelength that is the same as that of an incident light, and a small amount of a Raman scattering light having a wavelength shifted by Raman scattering is also included in the back scattering light. Intensity of the Rayleigh scattering light is generally about 1/100 of that of the incident light, and the Raman scattering light is as weak as about 1/10,000 of the Rayleigh scattering light. The Raman scattering light contains a stokes light whose wavelength is shifted toward a long wavelength and an anti-stokes light whose wavelength is shifted toward a short wavelength with respect to the incident light. The Raman scattering occurs when the light entered into the optical fiber collides with Silica molecules. Since the amount of motion of the Silica molecules is changed depending on a temperature, the scattering amount depending on the temperature is changed. That is, intensity of the stokes light and the anti-stokes light depends on the absolute temperature. Accordingly, if a ratio between the stokes light and the anti-stokes light is obtained, distribution of temperatures in the lengthwise direction of the measured optical fiber can be obtained. At this point, only the anti-stokes light is affected by the temperature, and the stokes light is measured in order to compensate drift of the light source by measuring the scattering amount. In order to measure the distributed temperatures of the optical fiber by separating and extracting the Raman scattering light from the back scattering light of the measured optical fiber based on the principles described above, the optical fiber distributed temperature sensor is absolutely required.
FIG. 1 is a view schematically showing the configuration of a conventional Raman sensor system. The Raman sensor system shown in FIG. 1 measures a temperature by the distance using a Raman phenomenon. The Raman sensor system generally comprises a power supply unit, a laser diode, a pulse generator, an optical fiber circulator, a Raman scattering measurement filter, an optical detector, an analog-to-digital converter (ADC), and the like. If a pulse-modulated incident light generated by the laser diode is transmitted into the measured optical fiber, delay of light occurs depending on the distance. Accordingly, an input optical signal of a pulse form is rendered into a scattering signal having a back distance resolution. The scattering signal generated from the optical fiber is divided into scattering signals having different wavelengths using the Raman filter. The Rayleigh scattering signal having a wavelength the same as that of the incident light generated by the laser diode is received in a Rayleigh scattering region, and an anti-stokes scattering signal and a stokes scattering signal capable of measuring changes of a signal with respect to changes of temperature are received in an anti-stokes scattering region and a stokes scattering region. The Rayleigh scattering signal, the anti-stokes scattering signal, and the stokes scattering signal received in the scattering regions are individually detected by the optical detector, and the detected signals are converted into digital signals by the analog-to-digital converter.
However, incorrectness of intensity distribution of the back scattering light in the Raman sensor system has been raised as a serious problem. The incorrectness is not only affected simply by temperature, but also caused by local attenuation generated by physical disturbance in the optical fiber. Influence of the local attenuation needs to be removed in order to avoid measurement errors caused by the incorrectness of intensity distribution. The incorrectness is mainly caused by a wavelength difference ranging from 100 nm to 200 nm between the stokes light and the anti-stokes light and dependency on the wavelength of the incident light. In addition, optical fibers manufactured in different manufacturing processes have differential attenuation distribution. Furthermore, the differential attenuation further increases when the optical fiber is obstructed by bending, tension, compression, radiation, and chemical contamination. In the case of the bending and compression, the differential attenuation is relatively small and thus can be compensated by a Rayleigh wave or a stokes wave. In addition, gamma ray radiation in a nuclear structure is a typical reason of a temperature error caused by the differential attenuation. Although a few methods using Rayleigh and anti-stokes bands have been proposed in order to solve the problems, the problem of difference in the wavelength still remains, and thus the differential attenuation caused by the wavelength difference cannot be completely removed.
In order to solve the problem, a dual-ended method for automatically correcting the differential attenuation has been proposed. However, although this method has a lot of advantages when the measured optical fiber is damaged, there is a problem in that an optical fiber two times as long as the measured optical fiber and an additional distributed temperature sensor channel are required. Later, a method of using a dual-light source has been proposed in order to solve the problem of the dual-ended method. However, the method of using a dual-light source still has a problem in that an additional light source, an optical switch, and two optical detectors are needed.
Accordingly, it needs to develop a method of further simply and completely measuring an automatically corrected temperature using one light source and one optical detector.